1. Field of the Invention
The invention pertains to intermediates of the formula:
as well as a method of their preparation by reacting a thiol having at least two hydroxyl groups with a mono-unsaturated organic compound in the presence of a base catalyst. A polymerizable urethane acrylate oligomer or urethane methacrylate oligomer is formed by reacting a polyisocyanate with the intermediate. The polymerizable urethane acrylate oligomer or urethane methacrylate is blended with a polymerization initiator to form a composition which is useful in such applications as adhesives.
2. Description of the Related Art
Curable adhesive compositions are well-known in the art. In the past, many adhesives particularly anaerobic adhesives, have been rendered resistant to degradation at elevated temperatures by the inclusion of certain additives. For instance, U.S. Pat. No. 3,988,299 teaches a heat curable composition having improved thermal properties, which includes certain acrylate monomers and maleimide compounds.
While the addition to curable adhesive compositions of such compounds to render them resistant to thermal degradation provides reaction products with acceptable performance, it would be desirable to find alternative compounds to include in such formulations. Thus, there is an on-going search for additives to improve the thermal performance of reaction products of curable adhesives. In addition, it would be desirable from a commercial, economic, environmental, supply and regulatory standpoint to provide alternatives and/or replacements for maleimide-type materials for improving the resistance to thermal degradation of reaction products of radical-curable adhesive compositions.
It has been unexpectedly found that improved adhesive formulations can be prepared by preparing intermediates by reacting a thiol having at least two hydroxyl groups with a mono-unsaturated organic compound in the presence of a base catalyst. When this intermediate is reacted with a polyisocyanate reactive acrylate or methacrylate a polymerizable urethane acrylate oligomer or urethane methacrylate oligomer is formed. When blended with a polymerization initiator a curable adhesive is formed. When applied to a substrate or carrier and cured by heating or actinic radiation, the adhesive results in a product having improved impact toughness and/or impact resistance useful in a wide variety of applications. A composition of the invention includes polyurethane (meth)acrylate oligomers (prepolymers), and formulations thereof, which contain residues of modified hydroxythiols in the oligomeric chain. Within the context of this invention, the term (meth)acrylate means either an acrylate or a methacrylate. These polyurethane (meth)acrylate oligomers will have a wide range of properties beneficial to industrial adhesive and coating applications due to the presence of both thioether functionality, which improve chemical and thermal resistance, and other functionalities that are incorporated through the hydroxythiol modification. Examples of such functionalities are carboxylic acids or other highly hydrophilic groups for water reduction/dilution, anti-fog coatings, and adhesion; hydrophobic groups for water resistance, adhesion to plastics and high refractive index groups for optical applications; silicone bearing groups for decreased surface tension, improved impact resistance, and anti-graffiti coatings; and (meth)acrylate groups for increased crosslink density, strength, and hardness. Other functionalities added to the oligomers change their pigment dispersion characteristics and therefore make them useful in inks. The ability to add pendant functionalities without compromising the physical properties such as tensile strength, scratch and chemical resistance, is especially important for commercial applications. For example in antifog coatings, having a high scratch and solvent resistance while keeping the antifog properties are contradictory and challenging. Prior techniques require expansive nanoparticles to obtain the necessary properties. With the present method it is possible to obtain scratch and solvent resistant coatings with very good antifog properties.
The preparation of the inventive oligomers is by a two stage process with the first stage being the modification of the hydroxythiol with the desired functionality. This is carried out through Michael Addition reaction of the hydroxythiol with a composition containing both the functionality and units of unsaturation, particularly acrylate. The product will be a (poly)ol with the number of thioether and functional groups equal to the starting number of thiol groups. This product, alone or with other polyols, is then reacted by standard methods with polyisocyanates and hydroxy (meth)acrylates or with monoisocyanate-(meth)acrylates to produce the polyurethane (meth)acrylate oligomer bearing the thioether and desired functionality.